Another possibility is Kakashi. He has a wealth of knowledge and experience. He might time - travel to the Fourth Shinobi War to save his comrades who were lost. He could use his Sharingan and his strategic mind to change the course of the war. Also, his relationship with many of the key characters makes him a character who could have a significant impact if he were to travel back in time during that war.
Naruto himself could be a likely time - traveler. He has a strong sense of justice and a desire to protect his friends and the village. If he saw a way to go back in time and prevent the destruction and loss in the Fourth Shinobi War, he would probably take it. Plus, his chakra and determination might be what enables the time - travel in the first place.
Naruto himself could be a likely time traveler. He has a strong sense of justice and a desire to prevent tragedies. If he learned of a way to travel back in time to the clan wars, he would probably try to stop the bloodshed and promote peace among the clans. Another possibility is Sasuke. Given his past and his connection to the Uchiha clan, he might want to go back to the clan wars to change the fate of his clan, especially the massacre.
One popular original character might be a young, prodigious shinobi from a small village. They have unique abilities that set them apart, like the power to control the elements in a completely new way. Another could be a mysterious figure who shows up during the war and has a connection to an ancient shinobi clan that was thought to be extinct. And then there's the brave female shinobi who defies gender norms and becomes a respected leader on the battlefield.
Sasuke would be a likely character to interact with Naruto as a Time Lord. Their complex relationship in the original Naruto series would add an extra layer of drama in a time - traveling scenario. Another could be Kakashi. His wisdom and experience would make for interesting conversations with Naruto, especially when Naruto has the power of a Time Lord. And of course, Jiraiya. Given his close relationship with Naruto and his own adventurous spirit, he'd be very involved in Naruto's time - traveling adventures.
Sasuke could also be a character who breaks the time loop. He is highly intelligent and has a thirst for power and knowledge. If he realizes that he is in a time loop, he would likely be very intrigued and start investigating. His Sharingan might give him some insights into the nature of the loop and help him find a way to break it.
He would likely interact with his father, Minato. Since Minato was very active during the Third War, Naruto would have a lot to talk to him about, like his own experiences and the future. Another character would be Jiraiya. Jiraiya was also involved in the war and he was Naruto's mentor, so Naruto would surely seek him out. And of course, he might interact with the Third Hokage as well, to gain more knowledge about the past and how to handle the situation in the war.
Naruto himself is very likely to be central. Since he is the main character in the original series, any alteration of time would likely still revolve around him in some way. His journey and how it's affected by the time changes would be a major focus. Sasuke is also a prime candidate. His relationship with Naruto and his own quest for power and revenge could be greatly altered by time changes. Another one could be Kakashi. He has a lot of backstory and knowledge, and in an altered time, his role as a mentor or a ninja in general could be very different.
Maybe in the fanfiction, the time travel causes a mix - up of jutsu knowledge. A character from the future accidentally gives the wrong kind of jutsu knowledge to a past - self or another character during the Fourth Shinobi War. This then leads to new and unforeseen strategies being developed by the shinobi, either for good or bad, depending on how it's used. It could completely change the power dynamics in the war.
Rock Lee is often overlooked in some of the larger war - time stories. His unique taijutsu skills could be really interesting to explore in the context of the Fourth Shinobi War. He could take on some really tough opponents and show off his growth.
Obviously Naruto would be the main character since it's about him time - traveling. Sasuke would also likely play an important role. Maybe he's skeptical of Naruto's time - traveling claims at first but then comes to believe him and helps him change things. Kakashi could also be a major character, providing guidance to Naruto based on his own experiences.
I think Obito could be really interesting. His complex backstory and his role in the war were so crucial. You could explore his internal struggle further, like his lingering feelings for Rin and how that influenced his actions during the war in more depth.